Cool Beans: Earth-Sized Planet in the Habitable Zone Discovered Very Near our Own Solar System

It's in the outer limits of the habitable zone, though. The cold part of the zone, like Mars. But it's more massive than Mars (more massive than Earth, in fact) so it could hold more of an atmosphere and thus be warmer.

Kepler-186f actually lies at the edge of the Kepler-186 star's habitable zone, meaning that liquid water on the planet's surface could freeze, according to study co-author Stephen Kane of San Francisco State University.

Because of its position in the outer part of the habitable zone, the planet's larger size could actually help keep its water liquid, Kane said in a statement. Since it is slightly bigger than Earth, Kepler-186f could have a thicker atmosphere, which would insulate the planet and potentially keep its water in liquid form, Kane added.

The planet orbits a red dwarf, much colder than our own sun, but the planet is much closer to it (and so is within the smaller star's smaller habitable zone).

This is kind of interesting. I know, vaguely, that a planet's atmospheric make-up depends on its mass. Mass determines not just how much gas a planet will hold in its atmosphere, but which gases, specifically. I believe it's easier to hold heavier gases, and harder to hold lighter ones (like hydrogen and helium).

Mars, being quite a bit less massive than earth, can't hold oxygen or nitrogen.

This planet, being just about earth's mass (1.1 earth-masses) could. But anyway, here's the interesting part: You can't go much more over earth's actual mass before a planet will begin trapping hydrogen and helium (rather than losing grip on these light atoms and letting them slip into space), and thus become not very earth-like at at all.

"What we've learned, just over the past few years, is that there is a definite transition which occurs around about 1.5 Earth radii," Quintana said in a statement. "What happens there is that for radii between 1.5 and 2 Earth radii, the planet becomes massive enough that it starts to accumulate a very thick hydrogen and helium atmosphere, so it starts to resemble the gas giants of our solar system rather than anything else that we see as terrestrial."

So "earth-like" is a very, very narrow range as far as mass -- say, I don't know, 0.8 earth masses to 1.5 earth masses -- and as far as distance from star.